Method of coating masses of small copper-bearing aluminum articles



Patented Mar 9, 1948 METHQD or COATING MASSES .OF SMALL COPPER-BEARINGALUMINUM ARTICLES William F. Speer, Leonia, N. J., assignor to AluminumCompany of America, Pittsburgh, Pa., a corporation of Pennsylvania NoDrawing. Application August 31, 1943, Serial No. 500,663

- 1 Claim. -l

This invention relates to the provision of aluminum articles with aprotective coating of aluminum oxide, and it is especially concernedwith the anodic treatment ofaluminum articles in bulk to produce thereona coating consisting substantially of aluminum oxide. Such coatings arereferred to herein as oxide coatings, and the term does not include thevery thin film of oxide formed on aluminum surfaces on exposure to theatmosphere.

The term aluminum as herein employed is intended to include bothaluminum and aluminum base alloys.

The process of providing aluminum articles with a protective coating ofaluminum oxide by anodic treatment is one well understood and widelypracticed in the art. In carrying out the process, the aluminum articleis made the anode in an electrolytic cell containing an electrolyte,and, under suitable conditions as to temperature, current density, andvoltage, a current is passed through the article to be coated for agiven length of time. When it is desired to anodically coat a largenumber of small aluminum parts, such as rivets or screws, and it becomesimpractical to connect each part separately to the source of current,the practice is to coat the articles in bulk by placing them in acompact mass in a suitable perforated container. The container is thenplaced in the electrolyte and through properly placed contacts, thearticles to be coated are connected to the source of current and thusmade anode. Despite the fact that the articles in the container arepacked together as tightly as possible to insure electrical contact andpassage of current through each article during the entire time ofimmersion in the electrolyte, occasionally the electrical contact withone or more oi the articles is broken, or may never be made. When thisoccurs, it is found that any copper present in solution in theelectrolyte tends to deposit galvanically on the surface of the articleswhich are not in the electric circuit. The material in these deposits,referred to herein as copper-containing material, may be made up eitherof copper or of copper-containing compounds. and usually varies in colorfrom brick red to black.

The presence of these adhering deposits of copper-containing material onthe aluminum surface is deleterious from the standpoint of theresistance of the surface to corrosion, since the surfaces exhibitingsuch deposition tend to pit and give other evidences of failure due tocorrosion much more rapidly than would otherwise be the case.

exhibit good resistance to corrosion, it has been necessary to resort toa careful inspection procedure so as to-elimin-ate those pieces uponwhich deposits of copper-containing material are found. This inspectionprocedure is, of course, tedious and expensive and it would be desirableif it could be eliminated.

The electrolyte whichis generally employed in the anodic treatment ofrivets and other articles treated'in bulk, and with which this inventionis concerned, is one consisting of an aqueous solution of sulfuric acid.The concentration of sulfuric acid may range from about 1 to '70 percent by weight, though-a concentration between about 10 and- 25 percent. by weight thereof is usually preferred. While this sulfuric acidelectrolyte is well adapted for anodically coating aluminum articles, itdoes not prevent the deposition of any copper present in solution in theelectrolyte upon the surface of any aluminum article which fails tomaintain proper electrical contact as an anode during: the time of itsimmersion in the bath. Copper is not ordinarily a component of thesulfuric acid electrolyte as originally compounded, but when theelectrolyte is used to anodize cop- 'per bearing aluminum base-alloyarticles, copper passes into the. solution. The quantity of copper whichis thus present in solution in the electrolyte is in most cases verysmall, representing buta fraction of 1 per cent by weight of theelectrolyte, since the bulk of the copper is plated out at the cathode.

It is. an object of this invention to provide a method of anodicallycoating aluminum articles in an electrolyte comprising an aqueoussolution of sulfuric acid, and having a quantity of copper present insolution therein, by which method nocoppe-r or. copper-containingsubstance will be deposited upon the surface of the aluminum articleimmersed in the electrolyte even though no current passes through thearticle during part or all of the period of its immersion. A furtherobject is to provide a sulfuric acid-containing electrolyte which,though copper be present in solution therein, may be employed toanodically coat aluminum articles in bulk without depositing said copperin any form upon the surface of any of said articles immersed in theelectrolyte.

I have discovered that an aqueous solution of sulfuric acid containingcopper dissolved therein may be employed to anodically coat aluminumalloy articles without danger of depositing copper or copper-containingmaterial upon the surface of any of the aluminum articles immersedAccordingly, when coating articles which must in the electrolyteprovided a small quantity of chromic acid be introduced into theelectrolyte. Chromic acid may be added either in the form of chromiumtrioxide (CrOa), or the addition may be made by using various chromateor dichromate salts, such as NazCrOr or NazCrzOv, which produce chromicacid when introduced into an acid solution. The chromic acid content ofthe electrolyte will hereafter, and in the appended claims, be expressedin terms of chromium trioxide, and in the event the addition is made inthe form of a salt, the'chromium trioxide content thereof is the factorwhich determines the amount of the salt to be used. ,The amount of thiscomponent which should be added to the bath in order to accomplish theintended purpose will vary according to conditions of operation. In somecases, as little as 0.2 per cent by weight chromium trioxide iseffective for the purposes of my invention, and in no instance has itbeen found necessary to add more than about 1 per cent by weight thereofin order to prevent the deposition of copper. Even though a lesserquantity of chromium trioxide would usually suffice, I prefer in allcases to employ about 1 per cent by weight of this component of theelectrolyte in order to avoid the making of the frequent additionsthereof which might otherwise be necessitated by reason of chemicalaction or dragout. Larger amounts may also be employed, however, butthis is not economical since such additional amounts are not needed toprevent the deposition of any copper-containing material. For example,an electrolyte having as high as 5 per cent chromium trioxide has beenemployed with good results. The chrcmic acid present in solution in theelectrolyte tends to be reduced during the anodic process, andconsequently it may be necessary to add further quantities of thiscomponent from time to time in order to maintain a given percentage ofchromic acid in solution.

The presence of the chromic acid in the electrolyte may require slightmodification of the operating conditions normally used with the sulfuricacid electrolyte. Ordinarily the chromic acid-sulfuric acid electrolytewill require a voltage approximately to 20 per cent greater than wouldbe the case were the bath to contain only sulfuric acid.

The following example will illustrate the manner in which this inventionmay be used. It was desired to provide a quantity of aluminum rivetsfabricated of a copper-magnesium-manganesealuminum alloy, which containsabout 4 per cent copper, among other constituents, with a suitable oxidecoating by anodic treatment in bulk. For comparative purposes, twoelectrolytes were employed in successivetests. On-e electrolyte had beenmade up of 16 per cent by weight of sulfuric acid, the balance water,while the other contained 16 per cent by weight of surfuric acid, 1 percent by weight chromium trioxide, the balance water. Each electrolytewas then employed to anodically coat a quantity of the rivets so thatcopper might be introduced into each solution. The rivets so treatedwere then discarded without comparative inspection. Thereafter, 12pounds of the bare rivets, placed in a suitable container, wereanodically treated in bulk in the sulfuric acid-water electrolyte. Inthis test the electrolyte was maintained at a temperature of about F.,and after the aluminum articles had been immersed in thesolution,connected as anode, current of about 6 amp/sq. ft. at 25 volts waspassed through the aluminum articles for a period of 40 minutes. Thealuminum articles were then removed from the solution and were examinedfor evidences of deposition of coppercontaining material. Approximately1 per cent of the rivets were found to exhibit a reddish to black stain,indicating deposition of copper-containing material. A subsequent testunder substantially the same conditions of operation was then conductedwith a second batch of 12 pounds of rivets in the sulfuric acid-chromicacid-water electrolyte, the only difference being that here the voltagewas increased to approximately 28 volts. -An examination of this secondbatch of rivets after treatment showed that no coppercontaining materialhad been deposited on the surface of any of the rivets.

' I claim:

The method of oxide coating a compact mass of small copper-bearingaluminum articles which comprises making said mass of articles the anodein an aqueous electrolyte consisting essentially of about 10 to 25percent by weight of sulfuric acid and about 0.2 to 1 percent by weightof chromium trioxide and having a fraction of 1 percent by weight ofcopper introduced therein from the articles treated, said chromiumtrioxide preventing the deposition of copper-containing ma terial on thesurfaces of articles that are out of electrical contact as anode duringthe anodic treatment.

WILLIAM F. SPEER.

REFERENCES CITED The following references are of record in the file ofthis patent:

FOREIGN PATENTS Duraluminumpublished June 13, 1927; page 1288.

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